1. Field of the Invention
The present invention relates to a fixing device that fixes an unfixed image on a recording medium, and an image forming apparatus that includes the fixing device.
2. Description of the Related Art
A fixing device is used in image forming apparatuses, such as printers, facsimiles, and copiers. A typical and widely-used fixing device includes a pressurizing roller and a fixing roller. The fixing roller includes a heater, and fixes an unfixed toner image on a paper sheet (recording medium). The fixing roller and the pressurizing roller form a nip portion in between. The temperature in the nip portion higher because of the heater present in the fixing roller. When a recording medium having an unfixed toner image thereon passes through the nip portion, the unfixed toner on the paper sheet is heated and fixed permanently to the paper sheet. Accordingly, a fixed toner image on the paper sheet is obtained. Such a fixing device includes a temperature detecting unit that is positioned at a center portion of the fixing roller in the axial direction of the fixing roller. When the recording medium passes through the nip portion, the recording medium absorbs heat of the fixing roller, therefore the temperature of the nip portion decreases. When the temperature detecting unit detects that the temperature of the nip portion has decreased, the heater is turned ON to raise the temperature of the fixing roller thereby maintaining the temperature of the nip portion to a desired temperature (hereinafter, “fixing temperature”) at which an unfixed toner image is fixed on a recording medium. The heat is not lost so much at the edge portions of the fixing roller where the paper sheet does not pass over. As a result, the temperature of the edge portions gradually increases as unfixed toner images are fixed on paper sheets one after the other. This may shorten the life of the fixing roller. Furthermore, when an unfixed toner image on a wide-width paper sheet is fixed thereon immediately after unfixed toner images on paper sheets each having a width smaller than that of the wide-width sheet are continuously fixed thereon, the quality of the fixed toner image may deteriorate significantly at the edge portions of the wide-width paper sheet. Such drawbacks can be seen also in a fixing device in which one of the fixing roller and the pressurizing roller serves as a stationary member, or a fixing device in which at least one of the fixing roller and the pressurizing roller serves as a belt member. Members for fixing an unfixed toner image on a recording medium can be termed as “fixing member”, and members for pressurizing the recording medium can be termed as “pressurizing member”.
For better understanding, a fixing roller and a pressurizing roller are explained below as examples of the fixing member and the pressurizing member.
To deal with the above drawbacks, thermal fixing devices are proposed (for example, see Japanese Patent Application Laid-open No. H5-134575). FIG. 8 is a cross section of a typical fixing device along a width direction. The fixing device includes the fixing roller 1, and a pressurizing roller 2. FIG. 9 is a longitudinal cross section of a typical fixing roller 1. The surface of the fixing roller 1 includes a sheet-path portion 12 over which a paper sheet 5 having an unfixed toner image thereon passes, and no-sheet-path portions 13 (edge portions) over which the paper sheet 5 does not pass. The pressurizing roller 2 is pressed against the fixing roller 1 to form a nip portion through which the paper sheet 5 passes. The fixing roller 1 includes a first heating lamp 4 and a second heating lamp 4′. The heat of the first heating lamp 4 conveyed to the sheet-path portion 12 is larger than that conveyed to the no-sheet-path portions 13. Moreover, the heat of the heating lamp 4′ conveyed to the no-sheet-path portions 13 is larger than that conveyed to the sheet-path portion 12. The heat flux of the heating lamps 4 and 4′ (i.e., light distribution of each of heating lamps 4 and 4′) is different in different portions of the fixing roller 1 such that the temperature of the no-sheet-path portions 13 does not increase. Therefore, the fixing device can deal with paper sheets having a wide width and a narrow width. In this configuration, however, the fixing device must have many heating. Moreover, many temperature sensors are required to monitor the temperature of the heating lamps. These facts disadvantageously increase the cost and the size of the device, and complicate the method of controlling the fixing temperature.
Some fixing devices have a stationary member, instead of the fixing roller, that functions as a pressurizing member and has a small frictional resistance. The same disadvantage, as that in the case of a roller, can be seen even in the case of the stationary member.
FIG. 10 is a cross section of an example of a typical fixing device including heat pipes 11. Japanese Patent No. 2793978 discloses a conventional fixing device that easily realizes substantially uniform heat distribution on the fixing roller in the axial direction of the fixing roller without increasing the size of the device. The conventional fixing device also includes cylindrical and hollow heat pipes. The heat pipes are arranged around a metal core of a fixing roller, and extend in the axial direction of the fixing roller. The heat pipes are filled with liquid. The liquid circulates in the axial direction of the fixing roller thereby producing substantially uniform heat distribution on the surface of the fixing roller. This inhibits the temperature of a “sheet-path portion” on the surface of the fixing roller from decreasing and inhibits the temperature of “no-sheet-path portions” on the surface of the fixing roller from increasing.
However, a fixing roller including heat pipes is more expensive than a typical fixing roller. For this reason, if a fixing roller or a pressurizing roller that needs to be regularly replaced includes heat pipes, the maintenance cost of the fixing device also increases. To assure the durability of the expensive fixing roller including the heat pipes, a material used for the surface of the fixing roller disadvantageously has to be selected among a few limited materials.